Various types of networks have been proposed for next generation optical access network systems that transmit optical signals to subscriber terminals without converting the signals into electrical form. Such optical access network systems include, for example, E-PON (Ethernet (registered trademark) Passive Optical Network) developed based on B-PON (Broadband PON) that supports ATM frames and adapted to support Ethernet (registered trademark) frames, GE-PON adapted to support Gigabit Ethernet (registered trademark), and G-PON whose implementations vary among system vendors.
In the PON system which is a passive double star (PDS) network, the network is constructed using only passive optical components, and contains no active elements that perform optical-to-electrical conversion on the way as in an active double star (ADS) network. This offers the advantage of being able to construct an end-to-end optical network at low cost. Further, deployment of WDM-PON that uses wavelength division multiplexed (WDM) signals is also being studied because of its low per-bit cost.
FIG. 1 illustrates schematically the configuration of a prior art PON system. As illustrated, the prior art PON system includes a central office optical line terminal OLT, optical network units ONU1 to ONU32 as optical communication terminals one for each subscriber, and an optical coupler which causes an optical transmission line leading from the optical line terminal OLT to branch out at an intermediate point.
For downstream traffic from the optical line terminal OLT to the optical network units ONU1 to ONU32, a continuous wave optical signal, for example, at the 1.49-μm band, is used. The optical line terminal OLT transmits the downstream signal at a fixed bit rate common to the optical network units ONU1 to ONU32 while maintaining synchronization with the respective network units.
On the other hand, for upstream traffic from the optical network units ONU1 to ONU32 to the optical line terminal OLT, an optical signal, for example, at the 1.3-μm band, is used. The optical network units ONU1 to ONU32 each transmit the upstream signal when the downstream signal is received from the optical line terminal OLT. The upstream signal also is transmitted at a fixed bit rate common to the respective network units.
Patent document 1 listed below discloses an optical access system in which an optical service unit (OSU) is connected to a plurality of optical network units (ONUs) via a plurality of power splitters, wherein downstream signal light having a plurality of wavelengths is split for the each power splitters by a wavelength splitting means disposed between the OSU and the power splitters.    Patent document 1: Japanese Unexamined Patent Publication No. 2006-81014